1. Field of the Invention
The present invention relates to charge amplifiers for use in radiation sensors, and more particularly to such charge amplifiers that exhibit an increased dynamic range.
2. Description of the Prior Art
In radiation sensors, ionizing radiation generates free charges in an amount Q that is proportional to the energy of the ionizing radiation. Charge is typically quantified by using a low-noise charge amplifier, which performs an integration through a feedback capacitor C, thus converting the charge Q into a voltage V=Q/C.
In FIG. 1, a charge amplifier configuration 10 is shown, where vn2 represents a noise power from signal processing electronics following the charge amplifier; such as a filter, buffer, analog-to-digital converter, and the like.
In order to make the noise contribution from the processing electronics negligible compared to the signal voltage, which is determined by V=Q/C, a charge-to-voltage conversion gain 1/C is preferably maximized. This can be achieved by minimizing the value of C. Assuming a noiseless charge amplifier, the signal-to-noise ratio is given by
      Q    /          (              C        ⁢                                            v              n              2                        _                              )        ,and the minimum detectable charge is given by
      Q    min    ≈      C    ⁢                                        v            n            2                    _                    .      On the other hand, a saturation voltage of the amplifier, which is equal to the supply voltage VDD in an ideal case, limits the measurable charge Q to a maximum value QMAX=VDDC. An assumption is made that the virtual ground input of the amplifier is at 0V.
The result is that a dynamic range QMAX/QMIN of the configuration shown in FIG. 1 is independent of C and given by
      V    DD    /                              v                      n            ⁢                                                          2                _              .  Even if the dynamic range can be improved by filtering performed in the signal processing stage, the result is limited by technology and system constraints to typically no more than a few hundred, such as 100-300.
Therefore, it would be advantageous if the dynamic range in charge amplifier configurations could be increased to well above a few hundred once technology and system constraints, such as signal processing and associated noise, are taken into consideration.